Document the default certificate duration and process to change

[leochr]

By default, the Certificate requested by Liberty Operator is valid for 90 days (duration), but the rotation happens well before that. The default renew-before period is 1/3 of the duration, so the rotation will happen every 30 days by default.

The default 90 days duration can be changed by updating the value of certManagerCertDuration in the ConfigMap open-liberty-operator (in the namespace where the Operator is installed). This global configuration applies to all applications managed by Liberty Operator.

• Documented for OLO
• Documented (or referenced) for RCO
• Documented for WLO

[kabicin]

Ready for review
WLO: https://github.ibm.com/websphere/operator-docs/issues/525 and https://github.ibm.com/websphere/operator-docs/issues/524
OLO: #586
RCO: application-stacks/runtime-component-operator#635